A semiconductor device such as a power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is required to possess high dielectric breakdown strength and low ON resistance. These devices typically include a gate structure having a field plate electrode disposed within a gate trench that is used to realize the high dielectric breakdown strength and low ON resistance. However, acquiring both high breakdown strength and low ON resistance are demands which generally contradict each other. To increase the breakdown strength of the semiconductor device, it is necessary to increase the thickness of a field plate oxide film which insulates a semiconductor layer and the field plate electrode from each other. As a result, during the steps of manufacturing of the semiconductor device, warping of the wafer in excess of an allowable range may occur.